Laminar flow, including sheath flow, is a technique useful in a variety of applications, including bead/particle counting, flow cytometry, waveguiding, and fluid control. Sheath flow involves surrounding a central flow stream (the core) with a surrounding stream (the sheath), wherein fluidic (e.g., hydrodynamic) forces compress the core flow stream into a narrow region. This permits the counting, detection, and/or sorting of a sample in the core, such as particles, beads, cells, and the like. In particle counting and flow cytometry applications, the sheath prevents particles in the core from coming into contact with the walls of the channel, thus preventing adhesion and clogging. The sheath also serves to focus the particles or molecules into the center of the channel, allowing for easy counting or measurement through optical or other means.
Sheath flow is a type of laminar flow where a sheath stream surrounds a core stream, with substantial avoidance of mixing between the core stream and the sheath stream. Laminar flow can also be used with fluids of different refractive index to create a waveguide in the core or sheath stream in order to measure transfer of analytes from one stream to the other, or to control the rate of interaction between molecules in one or both streams for carefully controlled chemistry or analysis.
Typically, two-dimensional (“2D”) flow focusing approaches use one or more sheath flows to horizontally compress the sample flow towards the center of the flow cell or channel. With such an approach, spatial particle/bead distribution within the flow remains unaffected in the vertical direction. Three-dimensional (“3D”) flow focusing controls particle spatial distribution in the vertical dimension by further focusing the sample flow in the vertical direction with three-dimensional sheathed flow with the core stream surrounded on all sides. For “Lab-On-A-Chip” applications, a well defined and dimensionally stable narrow particle/bead flow stream is very desirable. Additionally, three-dimensional sheathed flow (also termed 3D fluidic focusing) mitigates wall flow effects and sample damage, and reduces detection errors due to multiple particle events.
Several approaches have been proposed to achieve 3D flow focusing including: optical gradient focusing [1], groove/chevron focusing [2] and acoustic driven focusing [3], as well as more traditional approaches [4,5] (citations corresponding to these reference numerals appear at the end of this specification). However, for applications such as flow cytometery, these require complex optical beam profiles, precision groove fabrication steps, and/or precision electrical electrode or PZT networks.
A need exists for simplified apparatus for flow focusing while allowing straightforward optical access.